Upgrades may easier for you if you have status on United, but I guess you have to conside how long they might survive if you plan to build up a stock of miles.

Bmi offer full miles earning on cheap Y tickets on UA, and, probably, the lowest threshold to *G. They may comp your emerald, which might make it easier for you to reach gold.

Don't forget, with any *G that's not UA, you get free use of the RCCs, even on a domestic ticket.

bmi also offer some of the best value miles redemptions with their part miles/part cash ticket deals. These can be booked to originate in the US - I've done it!